Ultrathin adhesive-free flexible printed circuit is the developing trend of the microelectronic industry. According to the binding manner of the substrate and the copper foil of the flexible circuit board, the flexible circuit boards can be categorized into the flexible circuit board with adhesive and adhesive-free flexible circuit board. According to the structure, the flexible circuit board can be categorized into the single-sided flexible circuit board, double-sided flexible board, multi-layered soft board, rigid-flex board, etc. Though the price of the adhesive-free flexible circuit board is remarkably higher than that of the flexible board with adhesive, parameters of the adhesive-free flexible circuit board, including the flexibility, the binding force of copper foil and substrate, the flatness of pad, etc. are much higher than those of flexible board good with adhesive. Lower the thickness of the copper foil, the better will be the folding resistance, which is beneficial for the bare chip to be attached to the flexible board (COF).
Currently, there are three methods for manufacturing the substrate of adhesive-free flexible printed circuit (FPCB):
1. Sputtering/Plating method: This method uses a polyimide (PI) film as the substrate. After a layer of metal is plated on the PI film using vacuum sputtering plating, electroplating is further conducted to increases the thickness of the copper layer. The advantage of this method is that an ultrathin two-layer soft board can be produced. The thickness of the copper film can be as low as 3-12 μm. Moreover, a soft board having two sides of different thicknesses can be produced.
2. Casting method: This method uses a copper foil as the substrate. The synthesized polyimide resin is extruded and coated on the copper foil which is in the shape of a roll by a die head. After being dried by an oven and imidization, a two-layer soft board is formed. This method is usually used for the single-sided soft board, but is difficult to be used for manufacturing double-sided soft board substrate.
3. Lamination method: This method uses a non-thermoplastic PI thin film as a core layer base film and uses a thermoplastic PI thin film as a surface layer base film. Next, a copper foil is coated. After a short time of hot rolling, a two-layer soft board is formed by imidization and high-temperature lamination processing.
In the above methods, both the casting method and the lamination method need the copper foil as the substrate. However, it is difficult to make the thickness of copper foil less than 5 μm. The requirement for an ultrathin FPC in the developing trend of the microelectronic industry is hard to meet.
Ultrathin FPC can be obtained by sputtering/electroplating method. However, due to a low surface roughness of the polyimide thin film and chemical inertness, the copper film formed by sputtering deposition usually has a low peel strength. Moreover, the technical threshold of this method is high and this method is still in a research stage.
In the year of 2014, science and technology daily and major websites reported and reproduced that the team of Professor Yang Zhenguo in the Department of Material Science of Fudan University newly developed a novel process for manufacturing double-sided flexible printed circuit board, which is “print-absorb-catalyst addition” process. The core of this new process is that dedicated ionic absorbing ink is developed autonomously. As a connecting layer between the resin-based board and the conductive plated copper, such ionic absorbing ink can catalyze the chemical plating reaction. Special groups in the ionic absorbing ink, amido, carboxyl, hydroxy etc. can significantly improve the interfacial adhesive force between the circuit board substrate and the plated layer. This method can provide a new approach for green, low-cost, massive production, roll-to-roll manufacturing of the flexible printed circuit. However, such method in which the ionic absorbing ink is used as the connecting layer between the resin-based board and conductive plated copper only replaces the glue (adhesive) in the process of flexible circuit board with adhesive. Essentially, the flexible circuit board with adhesive is still needed.
Patent document CN 101684554A discloses a chemical copper plating liquid of a polyimide thin film and a chemical copper plating method thereof. However, the exemplary result in this patent did not test the most critical parameter—the peel strength of the copper film and polyimide thin film. Actually, if the polyimide thin film is only cleaned and processed, no matter whether the vacuum sputtering plating or the chemical plating is applied, the peel strengths of the metal film and polyimide thin film cannot reach the standard.
Based on the above technical background analysis, it is confirmed that technical difficulty for preparing an ultrathin polyimide (PI) adhesive-free flexible printed circuit substrate is the peel strength problems of polyimide thin film and the copper film (sputtering method or chemical plating). However, this technical difficulty has not been well solved all the way.